If a microcomputer or the like is required to be shut down for extended periods of time for energy conservation, an external “timer” circuit (“sleep timer”) is generally used to generate a “wake up” signal at the end of the shut down period. This “wake up” causes the microcomputer to restart, continuing to execute its program from the point at which it shut down to conserve energy, rather than resetting to the beginning of its program.
When a microcomputer is so required to shut down for extended periods of time for energy conservation, however, it becomes impractical to implement a typical type of “watchdog” timer because such require frequent activity to prevent them from generating a reset which resets the microcomputer program to its beginning, and which, indeed, is only desired in the event that the microcomputer has actually failed. Such restart at the end of such shut down, is effected by the before-mentioned sleep timer.
Microcomputers and microcomputers used in embedded systems occasionally fail to operate normally for various reasons, ranging from unintended software defects to electronic noise. When they fail, there is no one around to restart them, as one starts a desktop computer when it crashes. For this reason, watchdog timers are often used, being capable of resetting the microcomputer if it is not itself reset within a certain period of time. The microcomputer is usually programmed to reset the watchdog timer, and does this so long as it is operating normally. If it fails, however, the watchdog timer eventually goes off and rights the situation. The watchdog timer may be part of the microcomputer chip itself, or it may be an external device.
This approach becomes impractical, however, if a microcomputer is required to shut down and remain idle for relatively long time intervals (e.g. minutes) between occasional periods of activity, in order to conserve battery power or some other limited power supply, as described, for example, in U.S. Pat. No. 6,229,988 issued May 8, 2001. Watchdog timers in such applications usually must indeed be reset every two seconds or so; or even more often.
It is to the solution of such problems that the present invention is primarily directed through a novel use of clocking signals.